Motor driver to match rpm of hall sensor input

Thread Starter


Joined Apr 25, 2016
Hi all

I’m new here with a fairly basic understanding of electronics and I want to get a bit of advice on a project

I have an old VW Jetta with a mechanical speedometer that works from a rotating cable driven from the final drive in the gearbox, I have recently changed the engine and gearbox to a much more modern setup and the problem is that the speedo in the new gearbox is electronic, the final drive rotates a hall sensor that is read by a more modern speedo in the car it came from

my idea is to make a circuit that will read the RPM from the hall sensor in the new gearbox and drive a motor at that speed, which will be connected to the old speedo cable that can drive the speedometer.

The other problem is that the drive on the new gearbox may not be the same as the old one so I also need to vary the speed of the motor in relation to the hall input, so I can calibrate it basically with the help of a sat nav

my thoughts so far are to attach another hall sensor to the cable drive motor, convert both hall sensor outputs to a voltage (somehow), introduce a POT to the drive motor hall sensor output so I can vary the speed and introduce the two voltages to an op amp comparator which output will feed a motor driver. so the whole thing will act as a feedback loop

my questions are as follows:

will this work?

how to I convert a hall sensor output into a voltage for the op amp?

is there a better way of doing this? (I imagine there is)

any help will be greatly appreciated


Joined Sep 17, 2013
Welcome to AAC!
I think it should work in principle, though the devil will be in the detail. Others here will probably recommend using a micro to do all the dirty work :).